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7. Paleont., 71(2), 1997, pp. 261-284
Copyright © 1997, The Paleontological Society
0022-3360/97/0071 -0261 $03.00
MAZON CREEK CYCLOIDEA
FREDERICK R. SCHRAM, RONALD VONK, AND CEES H. J. HOF
Institute for Systematics and Population Biology, University of Amsterdam,
Post Box 94766, 1090 GT Amsterdam, The Netherlands
Mazon Creek Cycloidea contain four taxa: Cyclus americanus Packard, 1885, Cyclus obesus, new species, Halicyne
max, new species, and Apionicon apioides, new genus, new species. We conclude, based on a cladistic analysis, that cycloids are
specialized maxillopodan crustaceans and a possible sister group to the Copepoda. They may havefilleda niche similar to modernday crabs.
ABSTRACT—The
INTRODUCTION
PACKARD, 1885, is among the most
common of Pennsylvanian arthropods from the Essex biota of the Mazon Creek area of northeastern Illinois. Packard's
original description employed only a single specimen from the
famous Lacoe Collection, now in the National Museum in
Washington. At that time, despite a lack of good illustrations
in the literature for the European species of Cyclus with which
to compare his fossil, Packard managed to relate those previously known species to his specimen, interpreting C. americanus
as a larva of some kind of horseshoe crab. However, Packard's
effort stands as only a single incident in a long history of confusion and debate over the affinities of these enigmatic arthropods.
Phillips (1835) described the first cycloid based on a single
example from the Carboniferous Limestone of Yorkshire, England, assigning his peculiar little nut-shaped species, Agnostus
C
YCLUS AMERICANUS
radialis, with radiating grooves and ridges, to the trilobites. Not
long afterward, de Koninck (1841) concluded that Phillips' specimen was not a trilobite and created a new genus, Cyclus, for it
and other material of his own from the Carboniferous of Belgium. To de Koninck, the genus Cyclus clearly possessed a symmetrical round to oval shell with depressed margins, anterior
paired ocular tubercles, and posterior longitudinal and radial
sinuous ridges. Nevertheless, de Koninck did not have a clear
understanding of Cyclus because later (de Koninck, 1842) he
erected a second species (C. brongniartianus) that Woodward
(1870) subsequently recognized as a trilobite hypostome.
Phillips' and de Koninck's confusion of their fossils with the
agnostid trilobites was not an isolated case. Quite independently,
von Meyer (1838) rather casually recognized a new species of
what he thought was a trilobite from the Triassic Muschelkaik,
naming it Limulus agnotus. He subsequently decided in 1844
that this species was neither a trilobite nor a Limulus and erected
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JOURNAL OF PALEONTOLOGY, V. 71, NO. 2, 1997
for it the genus Halicyne. However, this genus remained rather
vaguely diagnosed until von Meyer (1847) named a second,
somewhat smaller, sister species to H. agnota from the same
beds, H. laxa. Although these Halicyne occurred as steinkerns,
i.e., interior molds of the shell or carapace, the genus clearly
was about as wide as long, possessed a truncated front margin,
had a rather vaulted shell with its height being about h the
length, and displayed a wide, flat, posteriorly pointed margin.
So, by the middle of the 1800s two distinct cycloid morphotypes occurred: a small, nut-like, ridged "skull cap," viz., Cyclus-,
and a flattened "shield," viz., Halicyne. In 1857, von Seebach
collected three poorly preserved specimens from the Triassic
Lettenkohle of Thuringia near Weimar and named them Halicyne plana. However, subsequent authors largely overlooked
this work. Schafhautl (1863, p. 423) recorded a form similar to
Halicyne, which he named Carcinaspis pustulosus, with a highly
papillose surface and sculpted margin from the Upper Triassic
of the Alps. Both H. plana and C. pustulosus broadly conformed
to the flattened shield morphotype.
In 1868 and 1870, Henry Woodward began to describe cycloids in considerable numbers. Besides C. radialis, Woodward
recognized five species from the Carboniferous rocks of the
British Isles: C. harknessi, C. bilobatus, C. rankini, C. torosus,
C. wrightii and C. jonesianus, and these conformed in broad
outline to the morphotype of Phillips' C. radialis, i.e., small,
cap-like forms. All but the first of these, though, rather lacked
the distinctive radiating grooves and ridges, features which C.
radialis and C. harknessi shared. Because the non-radiating caplike forms resembled the shield-like genus Halicyne, Woodward
demoted Halicyne to a subgenus within the genus Cyclus. Woodward thus began the confusion among cycloid genera that persists to this day.
As we noted, Packard (1885, 1886) described the first American species of Cyclus, C. americanus, from the famous Pennsylvanian Mazon Creek deposits of Illinois. Packard compared
his little fossil to larval Limulus, but in this he merely followed
upon himself since his own earlier published commentary (Packard, 1872) treated Cyclus as a late larva of, or possibly an adult,
Limulus. The Mazon Creek fossil itself (see Figure 1.1, 1.2)
certainly preserved little that would have justified saying so.
Unwittingly, however, Packard introduced yet a third morphotype into the cycloid pantheon, not recognized as such at the
time. This morphotype possessed not only the flattened and
shield-like form evocative of Halicyne, but also had a rounded
or concave posterior margin and an anteriorly extended rostral
plate.
Woodward (1893) erected another species, Cyclus scotti, and
the first elucidation of the great array of preservational variations in Cyclus came from Woodward (1894). Peach (1883)
influenced Woodward in this regard, by first recognizing that C.
rankini preserved a ventral surface, and then describing a species
of his own from the Coal Measures of Scotland, C. testudo, that
supposedly had biramous limbs. From this work of Peach,
Woodward then recognized that 1) his species C. torosus in
reality preserved the ventral surface; 2) C. jonesianus had many
preservational variants; and 3) one preservational variant of C.
radialis displayed a broken carapace in such a way as to show
part of the radiating grooved and ridged dorsal surface of the
segments that lay beneath the shield. Woodward also speculated
for the first time on cycloid functional morphology, suggesting
that Cyclus had an enormously developed labrum with either
the mouth moved way posteriad and the leg bases serving as
jaws (as in Limulus), or the labrum developed as a sucking tube
(as in Argulus). Finally, Woodward's publication characterized
Cyclus for the first time as ". . . undoubtedly . . . crustacean"
x
(Woodward, 1894, p. 534). Woodward also added two new
species to the growing list, C. scotti (1893) and C. johnsoni
(1894), which matched the general rounded shield-like form of
C. americanus and C. testudo.
Reed (1893) described another nut- or cap-like form with
radiating grooves and ridges, C. woodwardi. Thus by the turn
of the century, the British Cyclus assemblage exhibited great
diversity.
In parallel with this work on Coal Measure Cyclus and Triassic
Halicyne, descriptions of an enigmatic array of smaller, highly
vaulted cap-like species appeared. Von Schauroth (1854) presented a small granulate shell, Hemitrochiscus paradoxus, from
Upper Permian rocks in Germany. Gemmellaro (1890) demonstrated a distinctly different, spinous form from the Permian
strata of Sicily, Oonocarcinus insignis, as well as a form more
akin to rounded shield-like Cyclus originally called Paraprosopon reussi. Stolley (1915) discovered in Triassic rocks of the
Alps and the Balkans a tiny, cap-like form, Cyclocarcinus serratus, and a very peculiar, possibly spinose species, Mesoprosopon triasinum.
Despite the fact that by this time three distinctively different
forms of cycloid occurred in rocks of either Carboniferous or
Triassic age, the general consensus viewed them as closely related species. As an example, Rogers (1902) described some
additional highly vaulted, cap-like, papillose forms from the
Pennsylvanian limestones of Missouri, Cyclus communis with
supposedly large compound eyes and C. per marginal us. Clearly,
Rogers attached no significance to the vaulting since he also
described some flattened Cyclus forms, C. packardi, C. limbatus
with distinctive spines on the margin, and C. minutus.
Woodward (1905) re-entered the field again with a short note
on C. johnsoni and C. rankini in which he asserted, with apparently little basis except for Peach's earlier interpretation, that
all cycloids had biramous limbs; he also reiterated his view that
cycloids were limuloid-like crustaceans (not mutually exclusive
terms at that time in history because Limulus, and even trilobites, were thought of as "crustaceous" in nature). Reed (1908)
described an Irish cycloid, C. simulans.
Bill (1914) noted specimens of Halicyne from the Alsatian
Buntsandstein, and Trauth (1918) also found Halicyne in Upper
Triassic rocks of the Alps. Neither of these authors formally
assigned their specimens to distinct species.
Hopwood (1925) finally tried to deal with the three distinct
morphotypes and re-separated Halicyne from Cyclus. However,
he focused on characters somewhat at odds with the original
diagnoses of the genera. Hopwood viewed Halicyne as a large
form with a bifurcate or bilobed posterior margin possessing
punctate ornament; whereas he perceived Cyclus as a small form
with a posterior median ridge that could bifurcate to enclose a
triangular area anteriorly and with lobate, ridged, nodular, or
papillose ornament. Hopwood sorted out all known species of
that time based on these characters. The genus Halicyne contained the species agnota, americana, johnsoni, limbata, packardi, permarginata, and scotti. The genus Cyclus contained the
species radialis, bilobatus, communis, harknessi, woodwardi, jonesianus, minutus, torosus and wrightii. Hopwood (1925, p. 308)
could not determine the affinities of H. laxa, and decided that
C. rankini was merely the ventral side of one of the other species.
He also believed that the affinities of the cycloids lay with Branchiura, parasitic crustaceans also known as the fish lice.
Miiller (1955) clearly re-established the differences between
the two genera. Returning to the work of von Seebach (1857),
and prompted by some new material, Miiller identified the truncated anterior margin and the pointed median, posterior margin
as the distinctive features of the genus Halicyne.